An oil separator that separates oil mist included in gas which is a target of treatment is known. For example, the oil separator disclosed in PTL1 separates oil mist using centrifugal force created by a rotating member arranged between the inlet and the outlet for the gas.
This oil separator has hermetically partitioned a separation space where a rotating member is housed and a drive space where a driving mechanism for rotating this rotating member is housed. The rotating member includes a rotary shaft and a plurality of separation discs that are layered along the axis direction of this rotary shaft. The rotary shaft is attached with the axis line being along the vertical direction and a driving mechanism for rotating the rotary shaft about the axis is attached at the lower end part of the rotary shaft. For example, a nozzle for injecting oil and a set of vanes to which oil is sprayed, or a motor is used in the driving mechanism.